Rifaximin

ABSTRACT

Amorphous rifaximin, methods of making it, and pharmaceutical compositions containing it. Also described are methods of converting amorphous rifaximin to crystalline rifaximin and vice versa.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to amorphous rifaximin, pharmaceuticalcompositions containing the same, processes for preparing amorphousrifaximin and to therapeutic uses and therapeutic methods of treatmentemploying amorphous rifaximin, or such pharmaceutical compositions,medicaments or products.

BACKGROUND

Rifaximin is a semi-synthetic, rifamycin-based non-systematicantibiotic. It is chemically termed as(2S,16Z,18E,20S,21S,22R,23R,24R,25S,26S,27S,28E)-5,6,21,23,25-pentahydroxy-27-methoxy-2,4,11,16,20,22,24,26-octamethyl-2,7-(epoxypentadeca-[1,11,13]trienimino)benzofuro[4,5-e]pyrido[1,2-a]-benzimida-zole-1,15(2H)-dione,25-acetate(I).

Rifaximin is used for treatment of travelers' diarrhea caused bynoninvasive strains of Escherichia coli.

Rifaximin was first disclosed in U.S. Pat. No. 4,341,785 which alsodiscloses a process for its preparation and a method for crystallizationof rifaximin using suitable solvents or mixture of solvents. However,this patent does not mention the polymorphism of rifaximin.

Canadian patent CA1215976 discloses a process for the synthesis ofimidazo rifamycins which comprises reacting rifamycin S with2-amino-4-methyl pyridine.

U.S. Pat. No. 4,557,866 discloses a process for preparation ofrifaximin, but does not mention the polymorphs of rifaximin.

U.S. Pat. No. 7,045,620 discloses crystalline polymorphic forms ofrifaximin which are termed as rifaximin α, rifaximin β and rifaximin γ.These polymorphic forms are characterized using X-ray powderdiffraction. Further this patent mentions that γ form is poorlycrystalline with a high content of amorphous component. This patent alsodiscloses processes for preparation of these polymorphs which involveuse of processes of crystallization and drying as disclosed in U.S. Pat.No. 4,557,866 along with control of temperature at which the product iscrystallized, drying process, water content thereof. Further, accordingto this patent, crystal formation depends upon the presence of waterwithin the crystallization solvent.

The above patent discloses rifaximin α which is characterized by watercontent lower than 4.5% & powder X-ray diffractogram having significantpeaks are at values of diffraction angles 2θ of 6.6°; 7.4°; 7.9°, 8.8°,10.5°, 11.1°, 11.8°, 12.9°, 17.6°, 18.5°, 19.7°, 21.0°, 21.4°, 22.1°;rifaximin β which is characterized by water content higher than 4.5% &powder X-ray diffractogram having significant peaks are at values ofdiffraction angles 2θ of 5.4°; 6.4°; 7.0°, 7.8°, 9.0°, 10.4°, 13.1°,14.4°, 17.1°, 17.9°, 18.3°, 20.9° and rifaximin γ which is characterizedby poorer powder X-ray diffractogram because of poor crystallinity. Thesignificant peaks are at values of diffraction angles 2θ of 5.0°; 7.1°;8.4°.

US2005/0272754 also discloses polymorphs of rifaximin namely rifaximin αform, rifaximin β form & rifaximin γ form characterized by powder X-raydiffractogram, intrinsic dissolution rates and processes of preparationof polymorphic forms of rifaximin. However, none of the above patentsdisclose a wholly amorphous form of rifaximin.

It is a well known fact that different polymorphic forms of the samedrug may have substantial differences in certain pharmaceuticallyimportant properties. The amorphous form of a drug may exhibit differentdissolution characteristics and in some case different bioavailabilitypatterns compared to crystalline forms.

Further, amorphous and crystalline forms of a drug may have differenthandling properties, dissolution rates, solubility, and stability.

Furthermore, different physical forms may have different particle size,hardness and glass transition temperatures. Amorphous materials do notexhibit the three-dimensional long-range orders found in crystallinematerials, but are structurally more similar to liquids where thearrangement of molecules is random.

Amorphous solids do not give a definitive x-ray diffraction pattern(XRD). In addition, amorphous solids do not give rise to a specificmelting point and tend to liquefy at some point beyond the glasstransition temperature. Because amorphous solids do not have latticeenergy, they usually dissolve in a solvent more rapidly and consequentlymay provide enhanced bioavailability characteristics such as a higherrate and extent of absorption of the compound from the gastrointestinaltract. Also, amorphous forms of a drug may offer significant advantagesover crystalline forms of the same drug in the manufacturing process ofsolid dosage form such as compressibility.

Consequently, it would be a significant contribution to the art toprovide an amorphous form of rifaximin having increased solubility, andmethods of preparation, pharmaceutical formulations, and methods of usethereof.

Objectives of the Invention

Therefore, it is an object of the invention to provide amorphous form ofrifaximin and a process for preparation thereof.

It is also an object of the invention to provide processes for theinter-conversion of amorphous rifaximin to crystalline rifaximin and theinter-conversion of the crystalline forms.

Another object of the present invention is to provide pharmaceuticalcompositions comprising an amorphous form of rifaximin.

Yet another object of the present invention is to provide therapeuticuses and therapeutic methods of treatment employing compositionscomprising amorphous rifaximin.

SUMMARY OF THE INVENTION

In one aspect, the invention provides amorphous form of rifaximin. Thismay be characterized by its powder X-ray diffraction pattern, as shownin FIG. 1. The amorphous rifaximin may be characterised by its FT-IRspectrum, as shown in FIG. 2.

In another aspect, the invention provides a process for preparation ofamorphous form of rifaximin.

In another aspect, the invention provides processes for inter-conversionof amorphous rifaximin to crystalline rifaximin and inter-conversions ofcrystalline forms.

In another aspect, the invention comprises pharmaceutical compositionscomprising amorphous form of rifaximin along with pharmaceuticallyacceptable carrier.

In another aspect, the present invention provides therapeutic uses andtherapeutic methods of treatment employing the compositions comprisingamorphous rifaximin.

The present invention provides amorphous rifaximin in bulk form, unlikethe prior art which discloses a mixture of amorphous and crystallinerifaximin, and which provides no disclosure as to how to prepare bulkamorphous rifaximin. The amorphous rifaximin is substantially pure withpolymorphic purity of 99% or more. Furthermore, it is substantially freeof any peaks of crystalline rifaximin.

In accordance with the invention, it is possible to obtain amorphousrifaximin which is substantially free of any crystalline rifaximin.

DETAILED DESCRIPTION OF THE INVENTION

The term room temperature used in present application refers to atemperature range between 25-30° C.

The term stripping in this application refers to removal of traces ofthe first solvent from residue by adding second solvent and distillingit to residue.

In one embodiment, the present invention provides amorphous form ofrifaximin Amorphous form of rifaximin of the present invention ischaracterized by its powder X-ray diffraction pattern. The XRPD of theamorphous rifaximin was measured on a Rigaku DMAX 2200 Ultima⁺ PC seriesX-ray powder diffractometer using a Cu K_(α) radiation source, and ischaracterized by its XRPD pattern as shown in FIG. I. The amorphousrifaximin can be characterised by its FT-IR pattern, as shown in FIG. 2.Amorphous rifaximin according to the present invention is convenientlyprepared by a process, which comprises reaction of Rifamycin S with2-amino-4-picoline in presence of a suitable solvent likedichloromethane, ethyl acetate, dichloroethylene, chloroform, in aninert atmosphere. All these solvents can be used alone or in mixtureamong them or with water in various ratios.

Further, iodine dissolved in suitable solvent like dichloromethane,ethyl acetate, dichloroethylene, chloroform, is added at roomtemperature to the above reaction mixture and then stirred. Further,suitable reducing agent dissolved in water, is preferably added to theabove reaction mass and stirred at room temperature and then cooled to10-15° C.

The reducing agent used preferably comprises at least one of ascorbicacid, isoascorbic acid, sulphur dioxide, dihydroxyacetone.

Further, pH of the reaction mass is adjusted between 1.5-2.5, preferablyto 2.0-2.2 under stirring. The reaction mass is preferably furtherstirred for 10-15 minutes and organic layer is separated. The separatedorganic layer is preferably washed with water, followed by washing with10% sodium thiosulphate or 10% sodium metabisulphite and finally washedwith water till pH of the organic layer is neutral. This separatedorganic layer is preferably further charcoalised, filtered, dried oversodium sulphate and concentrated under vacuum below 50° C. to residue.

This residue contains rifaximin and can be further treated to obtaincrystalline rifaximin (as in the prior art) or amorphous rifaximin (inaccordance with the invention).

In the prior art, to manufacture the form known as the β form, theresidue would be treated with a water miscible solvent, followed bydrying in air at 80-110° C. To manufacture the γ form in accordance withthe prior art, the residue is treated with an organic acid and water,followed by drying in air at 100-110° C. I think is best to maintain thetemperatures, if they are the working temperatures, as we cannot add theinformation after filing.

In accordance with the present invention, amorphous rifaximin isprepared by subjecting a residue containing rifaximin to a strippingstep, followed by mixing with a mixture of water immiscible solvents,followed by drying at a temperature below 40° C.-the amorphous rifaximinmay be recovered after the drying step. The drying can be carried out atthe temperature somewhat below 40° C., e.g., at room temperature (forexample 25° C.).

More specifically, the residue obtained is preferably stripped out todryness with suitable water immiscible organic solvent and the materialobtained is isolated by stirring with the same solvent used forstripping or a mixture of solvents, preferably at room temperature.Further, the solid is filtered, washed with same solvents and driedbelow 40° C. to get amorphous rifaximin. The suitable solvent used forstripping of the product is a water immiscible organic solvent selectedfrom n-heptane, n-hexane, di-isopropyl ether, dichloromethane,dichloroethylene, chloroform and ethyl acetate.

The schematic representation for preparation of amorphous rifaximin isas follows:

Amorphous rifaximin according to the present invention can becharacterized by various parameters like solubility, intrinsicdissolution, bulk density, tapped density.

Rifaximin is known to exist in 3 polymorphic Forms namely α Form, β Form& γ Form of which the α Form is thermodynamically the most stable.Hence, the amorphous form of rifaximin was studied in comparison with αForm.

Further, when intrinsic dissolution of amorphous rifaximin is carriedout against the α Form, it is observed that the amorphous rifaximin hasbetter dissolution profile than α Form which is shown in table below(this data is also shown graphically in FIG. 3):

Dissolution medium: 1000 ml of 0.1M Sodium dihydrogen phosphatemonohydrate+4.5 g of sodium lauryl sulphate.

Temperature: 37±0.5° C.

Rotation speed: 100 rpmParticle size: Amorphous rifaximin—11 microns

-   -   α Form of rifaximin—13 microns

% Release of Amorphous % Release of α Form of Time in minutes RifaximinRifaximin 15 1.1 0.8 30 1.9 1.8 45 2.9 3.0 60 3.7 4.4 120 8.1 11.0 18012.6 18.0 240 16.6 24.6 360 24.7 38.7 480 32.0 47.5 600 39.5 52.7 72046.4 56.4 960 60.4 62.9 1200 72.9 67.8 1400 83.0 72.7

Amorphous rifaximin exhibits bulk density in the range of 0.3-0.4 g/mland tapped density is in the range of 0.4-0.5 g/ml while the α Formrifaximin exhibits bulk density in the range of 0.2-0.3 g/ml & tappeddensity is in the range of 0.3-0.4 g/ml. These higher densities ofamorphous rifaximin are advantageous in formulation specifically intablet formulation, for example, it gives better compressibility.

Another aspect of the present invention is to provide conversion ofamorphous rifaximin to crystalline γ form rifaximin which comprisesdissolving amorphous rifaximin in an organic solvent, heating preferablyto 40-60° C. and stirring the reaction mixture to get clear solution. Tothis organic solution, water may be added gradually preferably at 40-60°C. and stirred. The reaction mass may be cooled gradually to roomtemperature and stirred. The resulting solid may be filtered and washedwith mixture of organic acid and water. The solid may be further washedwith mixture of organic acid and water and then with water. The washedsolid is dried at 100-110° C. to yield rifaximin γ form.

Preferably, the organic solvent used for dissolution and washing, isorganic acid. The organic acid preferably can be acetic acid or formicacid.

Another embodiment of the present invention is to provide process forthe conversion of amorphous rifaximin to crystalline β form rifaximinwhich comprises dissolving amorphous rifaximin in an organic solvent,heating preferably to 40-60° C. and stirring the reaction mixture to getclear solution. To this organic solution, water may be added graduallyat 40-60° C. and stirred. The reaction mass may be cooled gradually toroom temperature and stirred. The resulting solid may be filtered andwashed with mixture of suitable organic solvent and water. The solid maybe further washed with mixture of organic solvent and water and thenwith water. The washed solid is dried at 80-110° C. to yield rifaximin βform.

Preferably, the organic solvent used for dissolution and washing,comprises at least one of water miscible solvents preferably acetone,acetonitrile, C₁₋₄ alcohols.

Another embodiment of the present invention is to provide process forinter-conversion of γ form of rifaximin which comprises dissolvingcrystalline rifaximin in suitable solvent, heating preferably to 40-60°C. and stirred. Water is added, preferably dropwise, preferably at40-60° C. to the above mixture under stirring. The resulting mixture maybe cooled gradually to room temperature and stirred. The solid obtainedmay be filtered, washed with mixture of solvents. Further the abovesolid may be washed with water, dried at 80-110° C. to yield β form ofrifaximin.

Suitable solvent used for dissolving and washing the product is selectedfrom water miscible solvents selected from group comprising of acetone,acetonitrile, lower alcohols or mixtures thereof.

Yet another embodiment of the present invention is to provide conversionof crystalline form of rifaximin to amorphous rifaximin which comprisesdissolving crystalline rifaximin in suitable solvent at room temperatureand filtered. The filtrate may be washed with suitable solvent mentionedabove and the solution may be concentrated, preferably under vacuum,preferably at 40-60° C. to get residue. The residue obtained is strippedout with suitable water immiscible organic solvent and then stirred inthe same solvent or mixture of solvents used for dissolving at roomtemperature, filtered, washed with same solvent and dried below 40° C.to get amorphous rifaximin.

The suitable solvent used for dissolving and stripping the product isselected from dichloromethane, dichloroethylene, chloroform, n-heptane,n-hexane and diisopropyl ether.

Yet another aspect of present invention provides pharmaceuticalcomposition comprising amorphous form of rifaximin in combination with apharmaceutically acceptable carrier. In addition to active ingredient(s)the pharmaceutical composition of the present invention may contain oneor more pharmaceutically acceptable ingredients.

The composition of the present invention can be formulated into varietyof dosage forms, such as tablets, capsules, pills, caplets, lozenges,dispersible granules, dry powder syrup, ready to use suspension;parenteral dosage forms available in the art; various inhalationformulations; transdermal formulations, and the like. These formulationscan be prepared using processes known in the art.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

FIG. I is an X-ray powder diffractogram (XRD) of amorphous rifaximinmade in accordance with example 1 as described below;

FIG. 2 is an FT-IR spectrum of amorphous rifaximin, made in accordancewith example 1 described below;

FIG. 3 is a graphical representation of intrinsic dissolution ofamorphous rifaximin compared with the α Form of rifaximin

The present invention will now be further illustrated by the followingexamples, which do not limit the scope of the invention in any way.

EXAMPLES Example 1

Rifamycin S 100 g (0.143 moles), dichloromethane 300 ml and2-amino-4-picoline, 46.4 g (0.434 moles) were mixed at room temperatureunder argon atmosphere. Iodine 19 g (0.074 moles) dissolved indichloromethane 700 nil, was added dropwise in 30-45 minutes at roomtemperature. Reaction mixture was then stirred at room temperature for15-18 hours. L(−)Ascorbic acid 20 g (0.113 moles) dissolved in 100 mlwater was added. The mixture was stirred for 30-45 minutes at roomtemperature and then cooled to 10 to 15° C. The pH of the reactionmixture was adjusted to 2 using 12.5% dil. HCl solution. The mass wasstirred for 10 to 15 minutes, organic layer was separated and washed atfirst with demineralized water then with 10% sodium thiosulfate andfinally with water till neutral pH was obtained. The organic layer wascharcolized, filtered through hyflo, dried over sodium sulfate andconcentrated under vacuum below 50° C. The product was stripped out withn-heptane and crude material thus obtained was stirred with a mixture of20% dichloromethane and heptane [500 ml] at room temperature for 30-45minutes. The solid was filtered, washed with a mixture of 20%dichloromethane and n-heptane and dried under vacuum below 40° C. for10-12 hours to get amorphous rifaximin 100 g.

Example 2

Amorphous rifaximin (100 g) was dissolved in acetic acid (200 ml) at 50°C., stirred for 30-45 minutes and demineralized water (200 ml) was addeddropwise at 50° C. in 30-45 minutes. Stirring was continued at 50° C.for 30-45 minutes, cooled gradually to room temperature and stirred for2 hours. The solid obtained was filtered and washed at first with aceticacid-water 1:1 mixture then with 10% acetic acid-water mixture andfinally washed with water. The solid obtained was dried at 100-110° C.for 12-15 hours to get 62-65 g of rifaximin-γ-form.

Example 3

Amorphous rifaximin (100 g) was dissolved in formic acid (200 ml) at 50°C., stirred for 30-45 minutes and demineralized water (200 ml) was addeddropwise at 50° C. in 30-45 minutes. Stirring was continued at 50° C.for 30-45 minutes, cooled gradually to room temperature and stirred for2 hours. The solid obtained was filtered and washed at first with formicacid-water 1:1 mixture then with 10% formic acid-water mixture andfinally washed with water. The solid obtained was dissolved in Isopropylalcohol (310 ml) at 50° C. and stirred at 50° C. for 30 minutes.Demineralized water (310 ml) was added dropwise at 50° C. in 30-45minutes and stirring was continued at the same temperature for 30-45minutes. The mixture was cooled gradually to room temperature andstirred for 2 hours. The solid obtained was filtered, washed withIsopropyl alcohol-water 1:1 mixture and then with demineralized water,dried at 80-90° C. for 10-15 hours to get 40-45 g of rifaximin-β-form.

Example 4

rifaximin γ form (62 g) was dissolved in acetonitrile (310 ml) at 50° C.and stirred at 50° C. for 30 minutes. Demineralized water (310 ml) wasadded dropwise at 50° C. in 30-45 minutes and stirring was continued atthe same temperature for 30-45 minutes. The mixture was cooled graduallyto room temperature and stirred for 2 hours. The solid obtained wasfiltered, washed with acetonitrile-water 1:1 mixture and then withdemineralized water, dried at 80-90° C. for 10-15 hours to get 40-45 gof rifaximin-β-form.

Example 5

Crystalline rifaximin (40 g) was dissolved in dichloromethane (10-15volumes) at room temperature, filtered through hyflo and washed withdichloromethane (2 volumes). The solution was concentrated under vacuumat 50° C. The solid was stripped out with n-heptane and stirred inn-heptane (50 ml) at room temperature for 30 minutes. Finally the solidwas filtered, washed with n-heptane and dried under vacuum below 40° C.to get 35-38 g of amorphous rifaximin.

Example 6

Tablet composition containing amorphous rifaximin.

Excipient Quantity (mg/tab) Rifaximin amorphous 200.00 Colloidal silicondioxide 2.00 Disodium edetate 2.00 Hydroxypropyl methyl cellulose 10.00Microcrystalline cellulose 162.00 Purified water q.s. Sodium starchglycolate 20.00 Glycerol palmitostearate 4.00 Suitable film coating10.00

A solid oral pharmaceutical formulation according to the presentinvention can be manufactured by granulation process known in the art.

1. Rifaximin in an amorphous form.
 2. Rifaximin in an amorphous formcharacterised by the XRPD pattern shown in FIG.
 1. 3. Rifaximin in anamorphous form characterised by the FT-IR spectrum shown in FIG.
 2. 4.Rifaximin according to claim 1, 2 or 3 having a purity level of at least99 wt %.
 5. Rifaximin according to any preceding claim, essentially freeof crystalline rifaximin.
 6. Rifaximin according to any preceding claimhaving a bulk density in the range greater than 0.3 to 0.4 g/ml.
 7. Acomposition comprising at least 99 wt % amorphous rifaximin according toany preceding claim.
 8. A pharmaceutical composition comprisingrifaximin according to any one of claims 1 to 7 in combination with apharmaceutically acceptable carrier.
 9. Rifaximin according to any oneof claims 1 to 5, for use as a medicament.
 10. The use of Rifaximinaccording to any one of claims 1 to 5, for use in the manufacture of amedicament for treating bowel related disorders like irritable bowelsyndrome, diarrhea, traveler's diarrhea, microbe associated diarrhea,Crohn's disease, chronic pancreatitis, pancreatic insufficiency and/orcolitis.
 11. A method of treating diarrhea, comprising administering atherapeutically effective amount of rifaximin according to any precedingclaim to a patient in need thereof.
 12. A process for manufacturingamorphous rifaximin as defined in any one of claims 1 to 5 comprising:a) reacting Rifamycin S with 2-amino-4-picoline in presence of suitablesolvent b) adding iodine dissolved in suitable solvent to the abovereaction mass followed by reducing agent c) adjusting the pH of thereaction mass between 1.5-2.5 under stirring d) extracting with waterimmiscible organic solvent and concentrating the organic layer to form aresidue e) stripping the residue obtained in step d) with a waterimmiscible solvent or mixture of water immiscible solvents to residue.f) stirring the residue obtained in step e) with a water immisciblesolvent or mixtures thereof to get a solid g) Filtering the solid andwashing with the same solvent/s and h) drying the solid at a temperaturebelow 40° C.
 13. A process according to claim 12, wherein the solvent orsolvents used in step (e) is selected from one or more of: n-heptane,n-hexane, di-isopropyl ether, dichloromethane, dichloroethylene,chloroform and ethyl acetate.
 14. A process according to claim 12 or 13,wherein the solvents in step (f) are selected from at least two of:n-heptane, n-hexane, di-isopropyl ether, dichloromethane,dichloroethylene, chloroform and ethyl acetate
 15. A process forconverting amorphous rifaximin, as defined in any one of claims 1 to 5,to crystalline γ form rifaximin, said process comprising combining theamorphous rifaximin with a mixture of an organic acid and water, thendrying the rifaximin mixture at a temperature from 100 to 110° C. toyield γ form rifaximin.
 16. A process according to claim 15, wherein theorganic acid is acetic acid or formic acid.
 17. A process according toclaim 15 or 16, wherein prior to combining the amorphous rifaximin withthe organic acid/water mixture, the rifaximin is dissolved in an organicsolvent at 40-60° C., combined with water, allowed to cool, and filteredto produce a rifaximin residue which is combined with the organic acidand water.
 18. A process for converting amorphous rifaximin, as definedin any one of claims 1 to 5, to crystalline β form rifaximin, saidprocess comprising combining the amorphous rifaximin with a mixture of awater-miscible organic solvent and water, then drying the rifaximinmixture at a temperature from 80 to 110° C. to yield β form rifaximin.19. A process according to claim 18, wherein the water-miscible solventis at least one of acetone, acetonitrile, and one or more C₁₋₄ alcohols.20. A process according to claim 18 or 19, wherein prior to combiningthe amorphous rifaximin with the mixture of organic solvent and water,the amorphous rifaximin is dissolved in an organic solvent at 40-60° C.,combined with water, allowed to cool, and filtered to produce arifaximin residue which is combined with the water miscible solvent andwater.
 21. A process for converting a crystalline form of rifaximin toan amorphous form of rifaximin as defined in any one of claims 1 to 7,said process comprising dissolving the crystalline rifaximin in asuitable solvent, filtering the solution, washing the filtrate with asuitable solvent, and concentrating the residue at 40-60° C. to form arifaximin residue followed by the steps of: (a) stripping rifaximinresidue with a water-immiscible solvent, or a mixture ofwater-immiscible solvents; (b) stirring the residue obtained in step (a)with water immiscible solvents or mixtures thereof to get a solid (c)filtering the solid and washing with the same solvent/s and (d) dryingthe solid at a temperature below 40° C.
 22. A process according to claim21, wherein the solvent used in step (b) is selected from one or moreof: n-heptane, n-hexane, di-isopropyl ether, dichloromethane,dichloroethylene, chloroform and ethyl acetate.